State of the Planet

News from the Columbia Climate School

NYC’s Heat Waves: A Harbinger of Things to Come?

Heat wave over Queens. Photo: Chris Goldberg
Queens heat wave. Photo: Chris Goldberg

Temperature records were broken this year. JFK and LaGuardia airports in New York City recorded their hottest August, and their second hottest summers ever. Eight states in the Northeast and Mid-Atlantic experienced the hottest August in 122 years. And NASA reported that globally, July and August 2016 were the hottest months ever recorded since record keeping began around 1880. This followed 11 consecutive months that set new monthly records for warmth.

As global temperatures rise, many wonder if New York City’s heat waves this summer were a result of climate change, and if we will experience more of them in the future.

The heat waves, defined as three consecutive days with temperatures of 90° F or above, were caused by a heat dome over the region, a high-pressure system that trapped hot air in the atmosphere. Because of the heat index, which measures how hot it feels to humans, a 90° F day can actually feel more like 106° F. Higher temperatures increase evaporation and retain more moisture and humidity in the air, which makes it more difficult for humans to cool themselves through perspiration.

NYC's many buildings retain heat.
NYC’s many buildings retain heat.

In cities like New York, the effects of high temperatures are exacerbated by the urban heat island effect—the asphalt and concrete of city buildings, streets and other structures absorb rather than reflect the heat, restrict airflow and can cause cities to be 1.8 to 22°F degrees hotter than surrounding areas. Nighttime temperatures too are generally higher than those of surrounding areas, since the buildings and streets retain the heat.

Heat waves can pose major health risks, especially since very hot days also bring increased ground-level ozone concentrations and pollutants. Children, the elderly and people with cardiovascular and respiratory issues are most vulnerable; that vulnerability is also affected by socio-economic factors such as economic status, education, the quality of housing and construction, availability of air-conditioning and access to aid.

Photo: Jenny Addison
Photo: Jenny Addison

Heat causes more deaths in New York City than any other weather condition. In addition to fatalities, high temperatures also cause heat exhaustion, characterized by dizziness, sweating, nausea, cramps and a weak rapid pulse; and heat stroke, accompanied by headache, lack of sweat, red hot dry skin, nausea, rapid pulse and loss of consciousness.

Until recently, scientists have been very careful to say that no single weather event can be blamed on climate change. Now, however, a new field of research called “extreme event attribution” is enabling scientists to estimate the extent to which climate change influences individual weather events.

One method for extreme event attribution hinges on the observational record to assess how the probability or intensity of events has changed over time; another uses computer modeling to compare a weather event that occurred with a reconstructed scenario of what might have happened without human climate intervention. Most studies combine both methods.

“In one common method, scientists perform many realistic computer model simulations, over long times (in computer years), of both the present climate, and the climate of a hypothetical, cooler world without human influence. In each climate, they count how often events occur that are similar to the one that happened in the real world. If they happen twice as often (say) in the simulated present climate as in the hypothetical climate without humans, then we say that human-induced climate change made the event twice as likely as it would have been otherwise,” explained Adam Sobel, in an article in the Washington Post. Sobel, a professor at Columbia University’s Department of Earth and Environmental Sciences, Applied Physics and Applied Mathematics and Lamont-Doherty Earth Observatory, was a member of the committee that put together the study on the new research.

The Coast Guard rescues people in Baton Rouge, LA Photo: IIP Photo Archive
The Coast Guard rescues people in Baton Rouge, LA Photo: IIP Photo Archive

(According to the National Oceanic and Atmospheric Administration, extreme event attribution determined that climate change increased the odds of heavy precipitation that caused the recent flooding in Louisiana by at least 40 percent.)

While every weather event has numerous causes, many of which are natural, scientists can now begin to determine how much climate change made an event more likely or more intense. But because the research is relatively new, results can be inconclusive if the models aren’t good enough, there is not enough historical observation, or the scientists do not have a clear enough understanding of how climate change influences a particular extreme weather event. Attribution works best with weather events related to temperature, such as heat waves.

Attribution studies haven’t been done on New York City’s recent heat waves, so it’s impossible to determine exactly what role climate change might have played. Once studies are done, however, Sobel believes they will show the fingerprints of climate change. “Most other studies done on major heat waves around the world, like the one in Australia in 2013, or the one in Russia in 2010, found that the events were more likely to have happened or were more severe compared to the pre-industrial climate,” he said. “It’s likely that this event [the NYC heat waves], when studied in its full complexity, will be found to have natural causes, but that human influence on the climate will also have made the heat waves warmer or the probability of reaching the temperature threshold more likely.”


According to the National Climate Assessment, between 1895 and 2011, temperatures in the Northeast increased almost 2° F. If we do not decrease our greenhouse gas emissions, by the 2080s we could see temperatures reach 4.5° to 10° degrees higher. Today New York City typically experiences two days a year above 100° F. The Union of Concerned Scientists projects that if greenhouse gas emissions remain high, we could see 60 or more days over 90° and 28 days with temperatures above 100° by the end of the century. That would make New York feel like South Carolina does today. If we do decrease emissions, the number could fall to 30 days over 90°, three to nine days over 100° and it would feel like Virginia. But regardless of which path we take, the frequency, intensity and duration of heat waves will likely increase, with more impacts if emissions remain high.

“What seems like a hot summer now will become normal, and if we’re not lucky, what seems like a hot summer now will actually feel cool in the future,” said Sobel. “What is today a heat wave will almost certainly be happening more frequently in the future—so that we may even have to change our definition of a heat wave. And the climate signal will get bigger, so the influence of climate change will be easier to trace.”

Between 2000 and 2006, over 600 New Yorkers died each year from heat-related deaths. A recent study by Columbia University Mailman School of Public Health scientists projected that over 3,000 people could suffer heat-related deaths each year by the 2080s if greenhouse gas emissions remain high and efforts are not made to adapt to higher temperatures. Even if emissions decrease, heat-related deaths could still reach over 1,500 each year. Parts of the South Bronx and East Harlem would be most affected because more poor live there, and there is less cooling greenery and parkland. The city’s growing aging population would also be especially vulnerable.

Volunteers painting a roof white. Photo: Community Environmental Center
Volunteers painting a roof white. Photo: Community Environmental Center

To deal with climate related risks like heat waves, the New York City Mayor’s Office of Recovery and Resiliency is trying to identify the city’s hot zones and determine how best to cool them. The Million Trees NYC initiative has already planted 500,000 new trees to provide shade, cool streets, clean the air and reduce flooding. It aims to have one million new trees planted across the five boroughs by next year. An initiative called NYC° Cool Roofs was launched in 2009 with the goal to coat one million square feet of rooftops with a white reflective coating each year. Replacing black asphalt with a cool roof coating reduces roof, internal building and urban heat island temperatures, and lessens energy consumption for air conditioning. The city’s Green Infrastructure Program is using green roofs, bioswales (depressions filled with vegetation) and rain gardens to cool buildings, clean the air and manage storm water.

To identify which populations are most at risk during heat waves, Jeffrey Shaman, associate professor of environmental health sciences at Columbia University’s Mailman School of Public Health, is conducting research trying to link how what goes on outdoors relates to what goes on indoors and how that impacts health. According to Shaman, most people in the developed world spend 90 percent of their time indoors, and a high percentage of heat-related deaths or heat strokes occur in the home, yet we monitor the environment mainly from the outdoors.

Photo: Patrick Stahl
Photo: Patrick Stahl

Twelve percent of New Yorkers don’t have air conditioning, and while landlords must provide heat in the winter, there is no requirement for them to provide air conditioning in the summer. “We are trying to understand how different heat and humidity levels vary in different types of buildings,” said Shaman. “How hot homes get in relationship to how hot it is outside, and how various subpopulations behave during heat waves due to economic circumstances or hardship. We have to identify which groups most need to be helped.”




Banner featuring a collage of extreme heat images.

Recent record-breaking heat waves have affected communities across the world. The Extreme Heat Workshop will bring together researchers and practitioners to advance the state of knowledge, identify community needs, and develop a framework for evaluating risks with a focus on climate justice. Register by June 15

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